Method of investment casting chaplet

ABSTRACT

A method of supporting a soluble insert structure within a mould die ceramic injection moulding (CIM) process, the method comprising the steps of: forming at least one chaplet that supports the soluble insert structure within the die, the chaplet being formed of a ceramic material that has substantially similar physicochemical properties to the main internal core structure; and positioning the chaplet to contact the soluble insert structure to the die, the soluble insert being spaced away from an edge of the mould die. The chaplet may comprise a refractory material or a combination of refractory materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This specification is based upon and claims the benefit of priority from United Kingdom patent application number GB 1901550.2 filed on Feb. 5, 2019, the entire contents of which are incorporated herein by reference.

BACKGROUND Field of the Disclosure

The invention relates to a method of supporting a core during lost wax or investment casting.

Description of the Related Art

The use of lost wax or investment casting is well known in the production of blades or vanes for use in gas turbine engines. Investment casting is an evolution of the lost-wax casting process in which the desired component is manufactured by injecting wax into the die before dipping it in a ceramic slurry to create an outer shell. The wax is removed and the ceramic shell is then fired to harden. The resulting shell has open cavities for the metal to pour inside in order to produce a product of the desired size and shape. Investment casting is an evolution of this process and is used to create hollow near net-shape metal components. This latter process is adopted as it allows for complex shapes to be reliably manufactured.

The process can be further used to create a series of complex internal cooling channels, which are desirable in modern turbine blade and vane design. To produce these components a ceramic core is manufactured separately usually using a ceramic injection moulding (CIM) technique. In this a ceramic material, usually silica, is suspended in an organic binder before being injected into a die cavity of the required shape. The ceramic core is positioned in the mould before the wax is injected and remains in the ceramic shell during the addition of the molten metal. The internal ceramic core can be subsequently removed at a later processing stage to leave a void where it was positioned. A ceramic core can be manufactured using soluble core manufacturing technology. A soluble insert is premanufactured and placed into a mould before the injection of the ceramic core materials. This soluble insert can subsequently be dissolved and removed. Methods for this process are disclosed in United Kingdom patent GB 2096523 B and U.S. Pat. No. 4,384,607. The use of soluble core manufacturing technology is desirable as it allows for complex re-entrant features to be manufactured.

One of the issues in using an investment casting process to produce hollow components is in the ability to maintain the ceramic cores in its correct position whilst undergoing the various processing stages. One method to overcome this is to use a chaplet these can be formed of plastic materials which can be mounted between the core and the outer die before the wax injection process. Chaplets are an effective method of supporting ceramic cores during injection of the wax pattern, however, the practice cannot be directly transferred to overcome issues of soluble insert movement during ceramic core manufacture. This is due to the fact that the plastic chaplet would leave a negative on the ceramic shape being formed. Whilst efforts could be made to repair or design for these negative features these issues limit the scope of plastic chaplet use.

Current methods, as discussed, present issues of removing marks left by the plastic chaplet during processing, which can affect the overall component and require further processing of the final product. Also they may not be able to maintain the soluble insert in the correct position to the required tolerances. When using soluble core manufacturing technology the current means of holding the soluble insert in position is through the use of prints, these are features of the soluble insert that extend beyond it and as such alter the soluble insert's profile and which are not beneficial to the core design. Ultimately, it is desirable to reduce the number of contact points between the die and the soluble insert so that large unsupported faces may be achieved, thus allowing for improved cooling designs related to turbine component technology to be created. However, in order to achieve this the soluble insert must be well supported at other locations to prevent movement, as any slippage or distortion of the soluble insert during the moulding process can lead to a non-conforming ceramic core. The object of the invention is to overcome or at least minimise one or more of these limitations in the investment casting process.

SUMMARY OF THE DISCLOSURE

In a first aspect of the present disclosure provides a method of supporting a soluble insert structure within a mould die ceramic injection moulding (CIM) process, the method comprising the steps of: forming a soluble insert structure, forming at least one chaplet that supports the soluble insert structure within the die, the chaplet being formed of a ceramic material that has substantially similar physicochemical properties to the soluble insert structure; and positioning the chaplet to contact the soluble insert structure to the die, the soluble insert being spaced away from an edge of the mould die.

The benefit of the present invention is that the chaplet becomes an integral part of the ceramic core. This allows the core and the chaplets to be removed at the same time—in the downstream ceramic core removal process after the investment casting process has been performed; this simplifies the investment casting process. The method may also limit the damage to the soluble insert and subsequent non-conformance of the ceramic core or the compromises that have to be added at the design stage to accommodate the positioning of the soluble insert. Consequently, the method allows for more complex internal structures to be produced, which in turn can lead to components having improved cooling flows within it.

The chaplet may comprise a refractory material or a combination of refractory materials.

The refractory material may be selected from silica, zirconia, alumina, alumina-silicate and combinations thereof.

The chaplet may be adhered to the surface of the soluble insert using glue.

The chaplet may be adhered to the surface of the soluble insert by contacting the chaplet with the surface of the soluble insert and melting either the surface of the chaplet or of the soluble insert.

The chaplet may be adhered to the die surface by contacting the chaplet with the die surface and melting either the surface of the chaplet or the die surface.

The chaplet is mounted using location features on the die.

The method wherein the resulting soluble core and the chaplet may have a bulk density in the range of around 1.3-2.5 g/cc.

The method wherein the resulting soluble core and the chaplet have a core that may have a porosity of around 20-40%.

The method wherein the resulting soluble core and the chaplet may be made of silica in the rage of around 30-98% weight.

The method wherein the CIM material for the soluble core and the chaplet comprises a binder in a range of around 10-25% weight.

The method may be used in the production of vanes, blades or seal segments for gas turbine engines.

The chaplet may be removed from the final cast at the same time as the core.

Multiple soluble cores may be used in the casting process.

The method may further comprise injection moulding an object around the soluble insert structure and the at least one chaplet within the mould die.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with reference to figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine; and

FIG. 2 is an example of a chaplet that is suitable for the method of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

With reference to FIG. 1, a gas turbine engine is generally indicated at 10, having a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, an intermediate pressure turbine 18, a low-pressure turbine 19 and an exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines both the intake 12 and the exhaust nozzle 20.

The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first air flow into the intermediate pressure compressor 14 and a second air flow which passes through a bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the air flow directed into it before delivering that air to the high pressure compressor 15 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high 17, intermediate 18 and low 19 pressure turbines drive respectively the high pressure compressor 15, intermediate pressure compressor 14 and fan 13, each by suitable interconnecting shaft.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan.

The production of turbine components for such engines requires complex castings. For this an investment casting process is used. Ceramic core components are positioned within a pocket in the die prior to the injection of wax. Soluble core manufacturing technology is used as they are able to create complex internal passageways that could not be formed conventionally because of pull planes in the die. Plastic chaplets are used to support the ceramic core components within the pocket of the die during the process to prevent movement of the cores. They are placed within the die, typically attached to the ceramic core to prevent any movement of the ceramic core during the wax injection process. The chaplets therefore act to maintain the minimum wall section within the casting. In this process any movement of the ceramic core could potentially lead to a faulty blade that has to be scrapped. In cases in which the design requirement dictates that only a small number of contact points can be used or there is a desire for a large unsupported surface then the ceramic core sections have a greater potential to move and/or break due to the lack of support. To avoid this, the choice and positioning of the chaplets is crucial; thus by having different sizes and shapes of these chaplets allows for these complex structures to be manufactured.

In manufacturing the blade, vane or seal segment a known standard process is used. When advanced designs are required soluble core manufacturing technology can be selected and a ceramic core of appropriate shape is formed through injection moulding a ceramic material into a die containing a soluble insert. At least one chaplet for supporting a soluble insert structure within the die is formed of a ceramic material. In this case the chaplets are formed from materials having similar physicochemical properties to that of the core. That is to say that they could have one of or more of similar density or dissolvability, physical or thermal properties. The chaplet and the core may be formed from ceramics comprising silica, zirconia, alumina, alumina-silicates or other refractory materials, and may be bound using a wax or a polymer based binder. Refractory materials are used as they are resistant to decomposition by heat, pressure or chemicals, which is desirable during the casting process. The soluble inserts are placed within the die along with the requisite chaplets at appropriate points to prevent the soluble insert form moving. The positioning of the chaplets relative to the soluble insert needs to take into account the position of the soluble core and the dimensional variation of the soluble core. The soluble core is formed of a using a ceramic injection moulding technique. The soluble core is then then removed from the die. It is then debound and sintered in a firing process. A final inspection step is performed on the soluble core before the core is used for the wax injection process. Prior to the wax injection step an inspection should be carried out to ensure the correct chaplets are used and that they are positioned in the correct positions. Ceramic material is injected into the die and subsequently wax can then be injected into the die around the core. When the wax has solidified, the wax body with the core structure inside is removed from the mould and dipped into a ceramic slurry to create a shell, the wax is then removed, and the shell is fired to hardened with the core and chaplets positioned inside. Molten metal is then poured into the shell to form a blade or other suitable component of the desired size and shape. After solidification of the metal, the shell is broken away leaving the cast blade with the ceramic core manufactured by the soluble core manufacturing process and the chaplets, which have become an integral part of the soluble core. These can, possibly, then be removed by leaching the core material away by dissolving it in an appropriate solvent. The solvent can be for example sodium hydroxide or potassium hydroxide, or any other suitable solvent as would be apparent to the person skilled in the art. Alternatively, the chaplets could be designed and configured to fall out of the casting after completion.

The formation of the chaplets themselves aims to meet two main criteria: Firstly, to minimise any surface imperfection formed on the ceramic core; and secondly, to ensure the chaplet retains its position between the soluble insert and the die surface. The chaplet will also leave a witness mark on the ceramic core surface, which needs to be minimised. This means that rather than conventional chaplets, which have wide bases and small contact points and are removed before the casting process, the desirable shape for soluble chaplets is for them to be point contacts. However, this is impractical to use all over as one of the sides needs to be adhered to either the die surface or the soluble insert. As such, it may be desirable to use point contact chaplets on critical areas and use of more complex chaplet design in non-critical areas. FIG. 2 shows an example of a non-point contact chaplet 20. The chaplet in this example is formed by injecting moulding a material with similar physicochemical properties to that of the ceramic core. The material is injection moulded into a die or sprue of suitable shape and size in a conventional manner. In the example shown a recess 22 is added to the chaplet to allow for a greater level of glue to be used. The materials used may include silica, zirconia, alumina, alumina-silicates or other refractory materials. They may also be bound using a wax or a polymer based binder. The chaplets are not limited to this shape but can be any suitable shape from point contacts to more complex shapes.

The physicochemical properties can include hardness, density, composition, solubility in differing solvents or any other suitable physicochemical property. This could mean that the chaplet and the core are made from the same or similar materials or have the same or slightly different solubility's in commonly used solvents. Similarities could mean that physical properties are within 20% of each other. Preferably they may be within 10% or within 5% of each other. For example, the core and chaplet may be made of silica in the rage of around 30-98%. Zircon may be present in the range of around 0-30%. Alumina may be present in the range of around 0-30%. Alumina-silicate may be present in the range of around 0-30%. A binder can be added in the injection formulation in a range of around 10-25%. Furthermore, minor additives may be included in the range of around 0-5%. This may provide a core that has a porosity of around 20-40%. The core may have a bulk density in the range of around 1.3-2.5 g/cc. The examples above are only an example of the materials that could be used, as the person skilled in the art will appreciate that other suitable materials could be used, such as replacing the silica in the blend with alumina. There are three options for securing the chaplet in position, these can be: adhering it to the soluble surface, adhering it to the die surface; or the use of location features. If adhering the chaplet to the surface of the soluble insert or die, then several options exist. The chaplet can be glued; these can be solvent based, temperature change based, or chemical reaction based adhesives. In this case a thin layer is preferred to minimise the effect on the surface of the part, as the glue will burn out during heat treatment and leave a negative on the surface. The chaplet may be melted and pushed in contact with the surface being adhered to and allowed to cool. Alternatively, another material can be melted between the two and used as an adhesive. The chaplet may be designed to enhance glue adhesion, for example this could be done by creating a recess in the chaplet to allow a greater level of glue to be used, as shown in FIG. 2. Another option, is for the chaplet to be self-locating on the soluble insert or the within the die by adding location features. This can be done for the use of a single chaplet at a crucial point or for multiple chaplets located around the core structure. An interconnecting passage may be required so that the chaplet does not contact either the surface of the soluble or that of the die. By not using glue there is a reduction in the number of processing steps used.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. 

We claim:
 1. A method of supporting a soluble insert structure within a mould die ceramic injection moulding (CIM) process, the method comprising the steps of: forming a soluble insert structure, forming at least one chaplet that supports the soluble insert structure within the die, the chaplet being formed of a ceramic material that has substantially similar physicochemical properties to the soluble insert structure; and positioning the chaplet to contact the soluble insert structure to the die, the soluble insert being spaced away from an edge of the mould die.
 2. The method as claimed in claim 1, wherein the chaplet comprises a refractory material or a combination of refractory materials.
 3. The method as claimed in claim 2, wherein the refractory material is selected from silica, zirconia, alumina, alumina-silicate and combinations thereof.
 4. The method as claimed in claim 1, wherein the chaplet is adhered to the surface of the soluble insert or the die using glue.
 5. The method as claimed in claim 1, wherein the chaplet is adhered to the surface of the soluble insert by contacting the chaplet with the surface of the soluble insert and melting either the surface of the chaplet or of the soluble insert.
 6. The method as claimed in claim 1, wherein the chaplet is adhered to the die by contacting the chaplet with the die and melting either the surface of the chaplet or the surface of the die where the chaplet is to be contacted.
 7. The method as claimed in claim 4, wherein the chaplet is mounted using location features on the die.
 8. The method as claimed in claim 5, wherein the chaplet is mounted using location features on the die.
 9. The method as claimed in claim 1, wherein the resulting soluble core and the chaplet have a bulk density in the range of around 1.3-2.5 g/cc.
 10. The method as claimed claim 1, wherein the resulting soluble core and the chaplet have a core that has a porosity of around 20-40%.
 11. The method as claimed claim 1, wherein the resulting soluble core and the chaplet are made of silica in the rage of around 30-98% weight.
 12. The method as claimed in claim 1, wherein the CIM material used to form the soluble core and the chaplet compromises a binder in a range of around 10-25% weight.
 13. The method as claimed claim 1, for use in the production of vanes, blades or seals for gas turbine engines.
 14. The method as claimed claim 1, further comprising injection moulding an object around the soluble insert structure and the at least one chaplet within the mould die. 